Oscar-winner Jennifer Hudson struggled to hold back tears while testifying at the trial of the man accused of killing her mother, brother and seven-year-old nephew in a jealous rage four years ago.

Hudson was the first witness called after prosecutors and attorneys for William Balfour finished their opening statements on Monday.

The singer and actress, who wore a subdued black dress and whose hair was wrapped in a bun, responded to prosecutors' questions softly but confidently and when asked to identify the defendant, poked her head around the judge's bench and pointed at Balfour.

"He's sitting right there," she said.

Her voice broke, though, as she started to describe her reaction when her sister, Julia Hudson, told her she was going to marry Balfour.

"None of us wanted her to marry him," she said, her voice cracking and struggling to hold back tears. "We did not like how he treated her," she said.

WIlliam Balfour. Photograph: AP

Balfour has pleaded not guilty to three counts of first-degree murder in the October 2008 slayings. He showed little emotion during opening statements, and slumped in his chair with his hand on his chin or elbows resting on the defence table.

Prosecutor Veryl Gambino said during opening statements that Balfour began threatening to harm the family after moving out of Hudson's family home.

"Those were not idle threats. … They were deadly warnings of what was to come," Gambino told the Cook County jury.

Defence attorney Amy Thompson offered jurors another take on the killings, telling jurors that police pinned them on Balfour because they felt pressured to make an arrest.

"As soon as that [that a celebrity was linked to the case] became known, they knew coverage would explode," Thompson said. "The police were on the hook. They had to find their man and find him fast."

Hudson, 30, was among 300 potential witnesses who could be called to testify during the trial, which could take up to a month.

Prosecutors have said that Balfour began threatening Hudson's family after becoming estranged from Julia Hudson. The couple's divorce was finalised last year.

Lead prosecutor James McKay has said that the day before the attack, which was Julia Hudson's birthday, he told her "If you ever leave me, I'm going to kill you, but I'm going to kill your family first." She didn't take him seriously, McKay said, because Balfour hadn't acted on the threats before.

Prosecutors say Balfour became enraged by balloons he saw at the home that he thought were from Julia Hudson's new boyfriend. She told investigators that on the day of the killings, she saw Balfour linger behind as she was driving off from home to her job as a school bus driver.

Prosecutors say Balfour went back inside the three-story house around 9am and used a .45-caliber handgun to kill Hudson's mother, 57-year-old Darnell Donerson, in the living room, and then shot her 29-year-old brother, Jason Hudson, twice in the head as he lay in bed.

He allegedly drove off in Jason Hudson's SUV with Julia Hudson's son, Julian King, inside. Authorities say he shot the boy nicknamed Juice Box in the head as he lay behind a front seat. His body was found in the abandoned vehicle miles away after a three-day search.

If convicted of at least two of the murder counts, the 30-year-old Balfour, on parole at the time of the killings after serving nearly seven years for attempted murder and vehicular hijacking, would face a mandatory life sentence.

Jennifer Hudson, the 2004 American Idol finalist and 2007 Oscar winner for her role in Dreamgirls, is expected to attend each day of testimony.

Judge Charles Burns has instructed jurors to set aside any sympathy for Hudson and decide a verdict strictly according to testimony. The panelists include a teacher, a trucker and two people who have had close relatives murdered.

Jennifer Hudson's stunning and soulful singing catapulted her from church choir to chart success and then on to Hollywood. But when three members of her family were murdered, it took all she had to keep going. She talks to Tim Adams about surviving grief